22 FORAMINIFERA OF THE CRAG. 
the cells differ in form and arrangement from those of Orditolites complanata; but the 
general form of the shell suggested Ordztolites for its genus. Orbiculina compressa is 
indeed an isomorph of the well-known O. complanata (more properly O. ordiculus) ; and 
in some instances they are with difficulty separated. 
In distribution this variety is associated with the typical species ; wherever the latter 
occurs abundantly we see the tendency in the bolder specimens to take the characters 
assigned to O. compressa. 
In the spiral form of Ordbciculina we recognise the type of a series of large-sized discoid 
Foraminifera common in tropical seas. The great diversity in appearance presented by 
different mature specimens, and the alterations which take place from time to time in the 
mode of growth of the shell, caused considerable confusion amongst the earlier writers, 
and were the cause of much unnecessary division into “ species.” D’Orbigny, in his 
‘Tableau Méthodique, and subsequently in the ‘Cuba’ Monograph, somewhat simplified 
the nomenclature, by uniting the species founded by Lamarck, De Montfort, and others, 
which were, in some cases, nothing else than the young, middle-aged, and adult of 
the same variety; but it was not until Professor Williamson, in 1851, published his 
researches on the minute structure of the shell that the correct relations of the forms was 
understood. No true specific difference exists between the specimens whose entire 
growth is on a flat spiral plan and those which ultimately assume a discoidal form by the 
alteration, after partial development, to a cyclical mode of increase; neither has any 
principle been found to account for this taking place. Another very variable character in 
the species is the condition of the chambers in regard to subdivision. In well-formed 
individuals each chamber is divided into chamberlets by transverse partitions ; but we 
frequently find, especially in small or poor specimens, the chambers simple, and free from 
any partition or constriction. ‘The surface of the shell normally exhibits a certain amount 
of surface-marking in the form of delicate parallel riblets, running in a transverse or 
oblique direction to the chambers, very similar to those of Pexeroplis ; but this, again, is by 
no means a constant character. 
Genus—Onrsrrotitrs, Lamarck. 
Navritus, Forskal. 
OrBITOLITES et OxBULITES (parte), Lamarck, 
Discouituus, Fortis. 
Discouttes, Montfort. 
MarcGinopora, Quoy and Gaimard. 
Sorires et AMenisorus, Lhrenberg. 
OxsiroLites, Defrance, D’ Orbigny, Curpenter, Parker and Jones, &e. 
